The present invention relates to the electrotechnical industry, precisely to adjustable transformers.
Widely known in the art are transformers regulated by the disconnection of part of the power winding turns with the aid of contactors or thyristors.
Transformers employing contactors are deprived of a smooth adjustment from the point of view of their characteristics and they are second to the two following types of transformers.
Most widely spread are transformers with series-connected opposing-parallel controlled valves exercising a pulse-position or a pulse-width adjustment. But the following disadvantages are characteristic to them:
(1) A low power efficiency which cannot be substantionally increased by connecting capacitor batteries and sharply goes down if the load power coefficient is cos .phi..sub.k &lt; 1, where .phi..sub.k denotes load power factor.
(2) The shape of voltage and current curves are considerably distorted which is greatly dangerous in case of big powers when high harmonics violate the normal operation of the connected consumers fail them.
(3) If phase adjustment is exercised by thyristors the load current should be reduced.
(4) Being of an equal price with the disclosed adjustable transformers they have poor reliability and in reality there are two elements of equal power in one device: a semiconductor adjuster and a transformer.
Damages to the control circuits of the thyristors (short-circuiting, discontinuity) causes failure in the device, the disclosed transformer being immune to this.
Known in the prior art are transformers with contactless switching of the power winding taps with the help of controlled valves having the advantages over the transformers discussed thereinabove only in the power efficiency and in a voltage curve shape. But their operation is still less reliable and their price is still higher since they integrate three elements: an adjustable transformer and two units of thyristors.
The automatic control circuit is still more expensive.
It is an object of the present invention to remove the above-mentioned deficiencies.
It is another object of the present invention to provide an adjustable transformer with advanced technico-economy parameters enabling to smoothly change the transformation ratio without distorting the sine shape of the output voltage.